I have to get retested again and it will be in 3 days on monday. I was thinking of using my old quick fix bottle and handwarmer (i covered it in plastic wrap after 4 hours of use) with my friends urine that I'll collect on the morning of the test. I was wondering how long does the handwarmer last, I know the box said 9 hours but I used it for 4 and had it wrapped for 3weeks. Can I just buy that heating pack at a brick and mortar store?